1. Field of the Invention
The present disclosure generally relates to image forming and, more particularly, to an image forming apparatus that optically detects adjustment toner images formed on image bearing members, and adjusts image forming conditions based on a result of the detection.
2. Description of the Related Art
To accommodate various types of recording materials, it is preferable, in an image forming apparatus, that adjustment toner images be formed and detected for adjustment of image forming conditions. The image forming apparatus includes an intermediate transfer member to which toner images are transferred from a plurality of image bearing members each having a photosensitive layer.
Conventionally, there has been a configuration in which an optical sensor that detects adjustment toner images is disposed to face an intermediate transfer member. In this configuration, however, repeated use of the image forming apparatus causes contamination of the surface of the intermediate transfer member and lowers the gloss of the surface of the intermediate transfer member. As the surface of the intermediate transfer member becomes darker in color, it becomes difficult to distinguish an adjustment toner image of black color from the surface of the intermediate transfer member. This means that it becomes difficult to detect the adjustment toner image of black color. To detect the adjustment toner image of black color even after repeated use of the image forming apparatus, it is preferable that adjustment toner images of other colors be detected on the intermediate transfer member and the adjustment toner image of black color be detected on an image bearing member. Since there is only a limited space between the image bearing member for black color and image bearing members for other colors, it is preferable that an optical sensor that detects adjustment toner images of other colors be disposed downstream of a transfer unit (for black color) to face the intermediate transfer member.
To detect the adjustment toner image of black color on the image bearing member, the optical sensor irradiates the adjustment toner image on the image bearing member with light. If the image bearing member is negatively charged, the potential of the region irradiated with light by the optical sensor is shifted in the positive direction. If a positive voltage is applied to the transfer unit when the adjustment toner image passes therethrough, the potential of the region irradiated with light by the optical sensor is further shifted in the positive direction. As a result, the polarity of the potential of the surface of the image bearing member may be reversed from negative to positive. This may cause image defects in the subsequent image formation.
Japanese Patent Application Laid-Open No. 2007-286445 describes a configuration in which an optical sensor that detects adjustment toner images is disposed to face an image bearing member. In the configuration described in Japanese Patent Application Laid-Open No. 2007-286445, to reduce traces of light irradiation from the optical sensor, a voltage of negative polarity higher than a discharge start voltage is applied to a transfer unit when a region where adjustment toner images on the image bearing member are irradiated with light passes through the transfer unit.
To reduce downtime, it is preferable that adjustment toner images of black and other colors be arranged in the width direction and simultaneously passed through the transfer unit.
However, if the method described in Japanese Patent Application Laid-Open No. 2007-286445 is used in which a voltage higher than the discharge start voltage is applied, a discharge occurs between an intermediate transfer belt and the image bearing member when the adjustment toner images of black and other colors simultaneously pass through the transfer unit (for black color). The discharge causes the adjustment toner images of other colors to be excessively retransferred to the image bearing member. This means that before being detected by an optical sensor on the downstream side, the adjustment toner images of other colors are excessively reduced in amount.